[1/3 header changes][metrics + grpc] Emit metrics for new headers handling behaviour. Hide actual changes under feature flag.

[biosvs]

Currently underlying implementation of headers handling wary significantly from one transport to another (docs/docs/headers-handling.md in this PR). Eventually, in one of the following releases, we want to make behaviour consistent and protective: filter inbound 'rpc-' headers, return error for attempting of 'rpc-' header setting.

Proposed changes are backward incompatible, so to identify the edges that are affected by the future changes, at first stage let's emit metrics.

Metrics: reserved_headers_stripped and reserved_headers_error metrics with "component": "yarpc-header-migration" constant tag and with source and dest variable tags

Part of https://github.com/yarpc/yarpc-go/issues/2265

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.33%. Comparing base (23c069f) to head (f7b3c8e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #2268 +/- ## ========================================== + Coverage 85.23% 85.33% +0.10% ========================================== Files 270 271 +1 Lines 15555 15624 +69 ========================================== + Hits 13258 13333 +75 + Misses 1876 1872 -4 + Partials 421 419 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.